1. Field of the Invention
This invention relates to an addressing method and apparatus which can be used for network communication via light transmission technology, and more particularly relates to an addressing method and apparatus for quantum information, including quantum communication and a quantum computation network system.
2. Description of the Related Art
Quantum information technology is a new scientific field which started in the 1990s. There are many problems being researched, and it is not applied on a large scale, so there will not be any open technologies related to this invention. According to the experience about classical network information technology, quantum network information technology will be the kernel technology of quantum communication. Comparing with classical communication and considering the characteristics of quantum communication, a quantum network system must satisfy these basic capability requirements:    1. multi-user
More than two users can exchange information and share resource at the same time.    2. extensibility
The number of users can increase and should not be limited theoretically.    3. independence of the users
This should include two parts: first, the existence and movement of any user only affect those users who exchange information with this user, not the whole network; second, the movement of any user should not be affected by other users except for the one who exchange information with this user, namely crosstalk.    4. Any user in the network should have one and only address or internal serial number (e.g. IP address in classical network), with this address, user can easily distinguish and connect to the user he want to communicate with, the user who is connected can also make sure which user he is talking to.
Special requirements of quantum network:    5. A network system should keep quantum coherence (except receiving and measurement), that means any decoherence process should be as weak as possible, and the system should not have any photoelectric or electrooptic conversion process except for receiving and measurement.    6. There should not be any amplification of the quantum signal inside the network system. This is the basic requirement of quantum no-cloning theorem.
Up to now, there are no quantum network framework theory and technology that accord with all the requirements above. The research on quantum key distribution over a network has made the biggest progress in this field; existing main technologies come down to:
1. tree topology network
This network has two working modes. One mode is, sending modulated single photons to multiple receivers randomly by fibered beam splitters, and then making quantum key distribution between sender and receivers. This mode needs a control center, all users have to share information with control center, that would be unendurable in secure communication sometimes, and besides, communication efficiency is in inverse ratio to the number of terminals in the network and descends when communication distance increases, that limits the extensibility of the network. The other mode uses wavelength addressing, that is, control center uses wavelength as address of receiver, makes quantum key distribution with any user. This mode solves the problem that communication efficiency decreases because of the beam splitters, but the security problem, sharing information with a control center, still exists. Another important problem is, users besides the control center can not communicate directly with each other, so a tree topology network is not a network proper.
2. ring topology network
This network links all users one by one to make one or more closed rings, any two users in the ring make secure communication by certain protocol with the help of control center and do not share any secure information with the center. In this network, all users must be in the ring, which limits the location of the user; the average communication distance between any two users is less than half of the ultimate quantum key distribution distance, and the more users it has, the smaller distance there will be between the adjacent users. This network has already has three modes, first one link with each user in series; an improved one uses a “space optical switch” to link many small rings to a big ring, every user can be on/off the big ring by using the “space optical switch”, this can achieve variable area quantum network, but in any case, there is only one pair of users can work at the same time; the third one use wavelength addressing, theoretically users can communicate with each other directly without center and work at the same time, but the limitation of the communication distance still exists, the number of wavelength must satisfy N=n*(n−1)/2, here N is the number of wavelength, n is the number of users, this limits the number of users in the network.
3. multi-particles entanglement network
This network can achieve quantum state transmission between multi-users, and is not only a quantum key distribution network, but also a generalized quantum information network. The problem is, the number of entangled particles has minus exponential relationship with product efficiency, and the number of users relates to the number of entangled particles directly, so the number of users is small, expansibility of the network is bad.
In a word, there is no network satisfy the basic requirements of quantum network at present.